This invention relates to alarm signal processing means particularly the processing of alarm signals generated by microwave fences and/or parallel wire intruder detection arrangements.
Two common forms of intruder detector systems used as external perimeter intrusion sensors are high frequency transmitter receiver combinations, known as microwave fences, and parallel wire capacitance change detectors known as electric field sensors. In the case of the microwave fence system a transmitter is set up facing an associated receiver and any disturbance of the received signal level as monitored by the receiver results in the generation of an alarm signal. It will be understood that a disturbance in the received signal is indicative of an intrusion into the area between the transmitter and the receiver. The size of the disturbance is proportional to the size of the intruder's radar cross-section as presented to the transmitter/receiver arrangement.
In the aforementioned parallel wire arrangement two sensor wires are supported on insulated posts parallel to each other and horizontal to the ground. They are connected to the primary wirings of a transformer and are then energised by an oscillator, the configuration being balanced by means of a potentiometer and a variable capacitor. When the circuit is balanced there is no voltage induced in the secondary winding of the transformer. However, when a person encroaches on either wire an imbalance will be induced into the system according to the proximity of the intruder to the sensor wire. This gives rise to generation of an alarm signal.
In each of these systems the alarm signal generated by the sensor devices has a value proportional to the amount of disturbance in the sensor's field. Thus, an erect walking or running person will produce a proportionately large disturbance when compared to a small animal, for example, a rabbit moving at the same speed. A problem with this type of sensor arrangement, however, is in distinguishing between a crawling human intruder and a small animal intruder, such as a rabbit, as both of these targets may cause approximately the same disturbance within the sensor field. Thus, if the sensor system is provided with a pre-set threshold value such that an alarm signal is only generated when the disturbance of the sensor field exceeds this predetermined value, the system will react to a crawling human intruder as it would to a small animal and, if the disturbance does not exceed the threshold will not generate an appropriate alarm signal.
Co-pending British Patent Application No. 2182517 proposes one way in which genuine signals indicative of an intruder penetrating a microwave fence may be distinguished from a signal generated by an animal. This is accomplished by providing two threshold detectors, one to detect a signal increase and a second to detect a signal decrease. An alarm signal will only be generated where a respective increase and decrease of the sensor field is monitored over a predetermined time period. In this way the disturbance caused by the presence of a human intruder in the detector zone which is characterised by initial small disturbances in received signal level of alternate positive and negative polarity followed by a relatively large decrease or increase in the signal level is detected. Whilst this system is successful in discriminating between disturbances produced by a human intruder and an animal it cannot distinguish when the cross-sectional area, to which the disturbance is proportional, of each intruder is approximately equal, for example, when a human intruder is crawling through the detection zone.